In completing an oil and/or gas well, a perforating gun is lowered into the cased borehole and the well is perforated by shooting holes or perforations through the casing, cement and into the hydrocarbon formation to permit the hydrocarbons to flow into the cased borehole and up to the surface. Perforating objectives include perforations of a desired size and configuration, penetration of the formation at least beyond whatever contamination may have occurred during drilling and cementing, prevention of further formation invasion and contamination during the perforating process, and maximum capacity to move hydrocarbons from formation to wellbore.
The effects of drilling fluids, cementing material and procedures, and perforating fluids seem disposed to harm the movement of hydrocarbons from formation to borehole more than benefit such movement. During drilling of the borehole, formation pressures are controlled by a weighted drilling fluid, filtrate and perhaps fines which invade the formation, interacting with in situ solids and fluids to create a zone of reduced permeability, and leaving on the face of the formation a low-permeability filter cake. The cementing operation also includes fluids and fines which invade and damage the formation.
The perforating gun sends metallic particles, charge debris, gas, borehole fluid, and perhaps small amounts of casing, cement and filter cake material into the newly opened perforations. This mass of metallic particles moving in a jet stream at a high velocity exerts an impact pressure in the magnitude of millions of psi and displaces material in front of the jet radially. As the jet pushes aside the formation, a portion of the rock is crushed and intensely compacted into a low-permeability zone. When all the jet particles have entered the perforation, penetration stops. The result in metal is compaction and distortion and the result in rock is compaction, fracturing, and crushing. Around the actual hole in the formation is an envelope of crushed and compacted rock and around that envelope is an envelope of fractured and compressed rock. Around those layers or envelopes is fractured but uncompressed rock and beyond that is undisturbed formation. If it is held in place by a plug of mud, charge debris and other material under pressure, the envelope of crushed and compacted rock reduces productivity of the perforation.
Practically all of the metallic particles from the jet may remain in the perforation along with the other charge debris. Gun debris has nearly zero permeability. Although some of such debris may flow out of the perforations, much of the debris may remain lodged in the perforations. Swabbing may not create sufficient differential pressure, or sufficient fluid movement, to dislodge the debris from all perforations. Even if much of the gun debris is removed, the compacted zone may still remain. Such a compacted zone can reduce flow from a perforation to 30% of what flow would be if the compacted zone were either not there, or had the same permeability as the undisturbed rock. Thus, a well could be producing only 30% of its potential.
Where only a few perforations are open flow channels, flow velocity in those channels is higher, thus causing a tendency towards formation erosion and water coning. Due to low deliverability, the economics of the well can also be adversely affected by plugged perforations.
Where the formation pressure sufficiently exceeds the borehole pressure, the newly formed perforations can be backsurged to wash out and repel the debris. By flushing out the perforations, the flow channel through the perforation is increased in size thereby increasing productivity.
Through-tubing perforators, i.e., small guns that can pass through tubing, are used in the prior art. However, in such operations, differential pressure is usually limited to a few hundred psi to prevent a high velocity fluid surge that would sweep the gun uphole and tangle the cable. Only a few hundred psi toward the wellbore is not sufficient to flush out the compacted zone. Further, the small through-tubing perforating guns provide only a limited penetration in extra hard formations. In conditions where two casing strings, two cement sheaths, and hard rock must be penetrated, through-tubing perforators are virtually useless in establishing effective flow channels. In any event, the hydrostatic "holddown" pressure these guns require reduces the differential pressure available for cleanup.
High-powered casing guns have sufficient power to produce a deep perforation tunnel that penetrates beyond the mud-damaged zone. However, the added charge of a casing gun enhances the compaction, fracturing and crushing of the formation. The high-pressure jet stream compacts the rock around the perforating tunnel, and forces mud and cement contaminants into the formation. This extensive formation damage reduces native permeability and severly limits production.
After conventional jet perforations, the well operator often cleans the perforation by swabbing. Such swabbing will clean only a few holes, and will not create sufficient differential pressure to dislodge the compacted rock surrounding the perforation tunnel. When additional cleanup is necessary, acid is forced through the plugged hole, subjecting both the cement and the formation to higher injection pressure and increasing the chance of cement damage and bottom water production.
Perforation quality is more important than shot density or penetration. To overcome such perforation damage in consolidated formations, emphasis has been placed on perforating with high differential pressure toward the wellbore so as to flush out debris and compaction. Much higher energy is needed to dislodge the zone of low permeability crushed rock lining jet gun perforation channels.
The applicants have, since 1970, been developing a "Tubing Conveyed" completion technique. This technique allows the formation pressure to backsurge the perforations immediately following the detonation of the gun. This provides a deep clean perforation with the crushed zone and debris completely removed. This technique includes lowering into the well a packer and casing gun on a substantially dry string of tubing, setting the packer, bleeding off the pressure trapped below the packer, opening the dry tubing to flow, detonating the gun, and immediately producing the formation. This technique provides deeply-penetrating perforations and immediate cleanup with high backsurge pressures and maximum flow. The resulting high backsurge pressure provides enough energy to give near ideal perforations.
In operation, one or more casing guns are assembled for lowering adjacent the formation. A mechanical firing head is placed on the top gun and the assembly is run on dry tubing below a conventional packer. A tubing sub with a radioactive tag in the bottom collar is installed above the packer for positive depth positioning. All components in the system are measured before going into the hole. The strapped distance from the top shot to the radioactive collar is recorded.
Once the approximate perforating depth is reached by tubing measurements, a gamma-ray or neutron logging tool is run through the tubing. This log locates the exact position of the perforating assembly with respect to the open hole log from which the perforating intervals were selected. The system is then placed in the exact position with tubing subs. A vent assembly is attached below the packer and is run closed. This allows the tubing to be run dry or with whatever fluid blanket is required.
When the packer is set, the vent assembly is then opened. This exposes a plurality of large diameter ports in the vent and the vent becomes a perforated nipple for the production tubing string. Opening the ports relieves the hydrostatic pressure below the packer and gives the differential pressure for perforating. Since in a new well the cased borehole is shut off from the formation, the bottomhole pressure is now at substantially atmospheric pressure. If required, acidizing can be done through the vent assembly.
At the surface the blowout preventer is removed, the wellhead is installed, the flow line is staked, a flare bucket is lit and the tree cap removed. A detonating bar is then dropped through the tubing to fire the gun. A mechanical firing system is generally used which eliminates the risk of electrically detonated blasting caps.
Immediately following detonation of the guns, the high differential pressure to the wellbore badksurges debris, mud filtrate and other contaminants, along with any rathole fluid, to the surface. Backsurging can be done at the highest controllable pressure differential, often as high as 5,000 psi. Natural cleanup usually is finished quickly.
If sand erosion is expected, back pressure can be held with a fluid cushion in the tubing string. Nitrogen gas (N.sub.2) may be used to hold back pressure during perforating and to "bring in" the well easily. Thus, the pressure differential may be predetermined by running the tubing into the well on closed, empty or partially fluid-filled tubing.
A tubing release is used to provide a safe, economical and dependable means of releasing the gun after perforation. This is important when a well requires stimulation, such as fracturing, and it is desirable to have the tubing open-ended so that ball sealers or other diverting agents can be used. Further, in dropping the guns, unrestricted gas flow may be allowed to the surface. In almost all cases, perforating is done with as much negative head as possible, surging the perforations to clean out damaging substances. Many times the guns are not dropped and are left in front of the producing zone to act as blast joints.
The Tubing Conveyed technique is designed for faster completion than conventional wireline methods. Tubing conveyed casing guns are designed for deeper penetrations and at multiple intervals in a single trip. The Tubing Conveyed technique avoids problems of the prior art such as wireline "stretch."
The Tubing Conveyed technique also assures safety. When the tubing is run, the packer is set, the blowout preventer removed, and the wellhead installed, all work at the surface is completed and tested for safety before the perforating guns are fired. These steps insure safety during all completions, especially those complicated by high bottomhole pressure or the presence of hydrogen sulfide. Prior art wireline methods require a delicate balance between expected formation pressure and casing fluid. Miscalculations when using through-tubing guns can result in the need to fish a tangled wireline. This may require pulling the tubing under adverse high-pressure conditions, and, if an overbalanced condition is used to perforate with a casing gun, some wells may "go on a vacuum," losing a large volume of fluid into the rock.
The Tubing Conveyed technique provides proper pressure differential which is needed to effectively clean perforations. Since all surface work is completed before the guns are fired, the differential pressure used to backsurge the perforation can be controlled. This permits cleaner wells and higher yields. The Tubing Conveyed technique may also be used in a highly deviated hole because the perforating assembly is lowered into the well on a tubing string and not a wireline.
The combination of hollow carrier gun for deep penetration and the flushing effect of 1,000 to 5,000 psi differential pressure produces near ideal perforations, i.e., long clean channels surrounded by formation that has not undergone permeability reduction by the penetrating media.
The Tubing Conveyed technique uses the raw power of virgin formation pressure to help solve the completion problems. The wells can be completed naturally without using extensive stimulation to overcome the damage from drilling, cementing, and perforating. Thus, wells can be completed in sensitive, problem formations where earlier completions were often impossible. Further, the Tubing Conveyed technique provides reliability, safety, speed, and overall economy that cannot be equalled by earlier completion methods. The technique is designed to immediately allow the safe release of formation pressure at maximum differential under the tubing. This backsurging cleans the perforation of mud filtrate, cement contaminants, and perforating debris, and allows each pay zone to produce at its full, natural capacity.
As indicated previously, formation pressures are often over several thousand psi and for safety reasons, as for example to prevent a blowout, a hydrostatic head is maintained in the cased borehole to provide a bottomhole pressure greater than the formation pressure. The hydrostatic head may be calculated by determining the weight of the column of mud in the casing. Although various means are provided to estimate the pressure of the formation, generally the hydrostatic head is maintained approximately 10% greater than that of the formation pressure.
However, in order to obtain a backsurge, the bottomhole pressure, i.e., the hydrostatic head, must be reduced to below that of the formation pressure. If the hydrostatic head were to be greater than the formation pressure at the time of well completion, well fluids in the casing would tend to flow into the formation, i.e., towards the lower pressure. One method of reducing the hydrostatic head is to displace the mud or other well fluid with a lighter fluid so as to reduce the hydrostatic head to a pressure less than the formation pressure.
As described earlier, in the Tubing Conveyed technique, the tubing string is lowered into the well substantially dry with the interior of the string being approximately atmospheric pressure. Upon setting the packer, the bottomhole pressure, caused by the hydrostatic head, is trapped beneath the packer. Unless that pressure is reduced, the bottomhole pressure will again be greater than the formation pressure preventing a backsurge.
One method taught by the prior art is to simultaneously open the dry tubing string at the time of perforation. Such a procedure has severe shortcomings. If the trapped bottomhole pressure is released suddenly at the time of perforation, a sudden pressure differential is created across the casing adjacent the formation as the trapped bottomhole pressure and formation fluids rush to the surface through the dry tubing string. This sudden pressure release causes a shock wave amounting to a kinetic force moving from the formation to the surface. Since the perforations through the casing are not large enough to take this shock force, the casing will, in most instances, collapse, ruining the well.
Further, the shock wave will tend to move the packer thereby causing the packer to lose its seal. Thus, a blowout could occur.
The preferred method is to vent the trapped bottomhole pressure below the packer prior to perforation. This release of the trapped bottomhole pressure permits the stresses, such as stress risers, in the casing to flow and distribute creating a static pressure differential across the casing rather than a dynamic pressure differential. Thus, the formation pressure becomes a static force around the casing rather than a dynamic force. By venting the trapped bottomhole pressure, the bottomhole pressure becomes substantially atmospheric pressure, creating a large static pressure across the casing. Upon perforation, the formation pressure is all vented through the perforations, permitting an enhanced backsurging.
The amount of time required to release the trapped bottomhole pressure varies with the formation pressure. In many wells, this trapped pressure may be released in a matter of seconds. For example, in the present invention, a bar actuated vent assembly is mounted on a 30-foot pipe section above the perforating gun. This pipe section is filled with clear fluids. As the bar drops through the tubing, it breaks kobes above the vent assembly to bleed pressure into the tubing string and then it engages the bar actuated vent assembly, and opens the vent assembly and tubing to fluid flow, thereby releasing the trapped bottomhole pressure. Because the pipe section is filled with fluid, the bar's descent is slowed due to the viscosity of the fluid. Thus, the bar's descent over the last 30 feet takes a second or two before the bar detonates the perforating gun. This time is sufficient to release the trapped bottomhole pressure and cause the bottomhole pressure to become atmospheric. The pressure differential across the casing then becomes static with a large pressure differential, i.e., the difference between atmospheric and the formation pressure. The greater the pressure differential, the better the backsurging and enhancement of well production.
It is necessary to have flexibility in the system. In some instances, it is desirable to have the tubing dry, and in others, to have a predetermined hydrostatic head in the tubing, or in others, to run the tubing in wet to control the well and then reduce the hydrostatic head in the tubing string for a predetermined underbalance.
Vent assemblies have multiple purposes. They can be used to keep the tubing string dry, they can be used as a perforated nipple, or they can be used to keep the tubing wet until one is ready to swab the tubing for perforation. It is of course important to insure that the casing does not collapse. Thus, in certain instances, such as that described above, the pressure must be bled off to open the vent assembly and then the vent assembly opened to release the trapped pressure below the packer.
It is preferred with blank casing having no perforations, to have kobes above the vent assembly which are clipped off as the bar drops through the tubing. Once the kobes are broken, small holes are opened in the tubing to permit the trapped pressure below the packer to bleed into the tubing string. The bar then travels on downward to open the vent and eventually fire the perforating gun.
However, where a producing well is being reperforated or old perforations are being surged, the cased borehole is open with the formation so that the bottomhole pressure and formation pressure tend to equalize. Thus, there is no differential across the casing and the kobes are eliminated to permit surging at the same instant as perforating. The objective is a controlled differential across the formation. In that situation, the bar is dropped, shear pins are sheared, the sleeve moves downwardly to open the vent assembly and the bar continues on to detonate the perforating gun. Thus, before the old perforations have had time to pressure up the entire system and raise the fluid level in the tubing string, the vent assembly is opened and the perforating gun is detonated simultaneously. At that time, the old perforations are surged and the new perforations are made with this underbalance practically at the same time.
The principal difference between a well that has never been perforated and a previously perforated well is that in the new well, the formation pressure cannot get into the casing and thus, it is necessary to open the vent assembly prior to perforating. However, in the case of an old well which has already been perforated, if the vent assembly is opened a substantial predetermined time prior to perforating, the formation pressure is vented into the borehole and into the tubing string so as to lose the differential pressure across the old perforation. In such a case, there would be little or no backsurging. Thus, one would want to keep the tubing string closed and dry as long as possible so that the new perforations could be made and the old perforations surged when there is a maximum pressure differential. In such a case, casing collapse would not be a problem.
Also, it may not be desirable to surge even a new well where there is unconsolidated sand. In such a situation, it may be desirable to only have a 500 psi differential.
Various vent assemblies of the applicant are disclosed in the prior art. Packer actuated vent assemblies are shown in U.S. Pat. Nos. 3,871,448; 3,931,855 and 4,040,485. U.S. Pat. No. 4,151,800 discloses a wireline actuated vent assembly and U.S. Pat. No. 4,299,287 discloses a bar actuated vent assembly. A pressure actuated vent assembly is the subject of U.S. patent application Ser. No. 166,547, filed July 7, 1980.
In U.S. Pat. No. 4,299,287, there is disclosed the combination of a gun firing head and a vent assembly which are sequentially moved to the operative position in response to impact of a free falling bar thereagainst. The vent assembly includes a sliding valve element which covers a port, and which is engaged by the falling bar and moved to the open position. The sliding valve element has no positive latch and therefore continues to fall toward the bottom of the hole with the bar. The bar ultimately impacts against the gun firing head, thereby detonating the shaped charges of the gun. Also, no means is provided to determine whether the vent assembly was opened or stayed open.
It has been found undesirable to allow the sliding valve element of the vent assembly to be carried downhole by the traveling bar for the reason that the valve element occupies a considerable amount of the annular area between the bar and the tubing string, thereby unduly reducing the velocity of the downward travel of the bar so that the bar does not always impact against the gun firing head with sufficient force to detonate the gun. This is especially so when debris partially obscures the trigger device of the gun firing head.
Moreover, it is undesirable to have the sliding valve element of the vent assembly disconnected from the tool string structure and translocated further downhole in the borehole because one can never be absolutely certain of the location of this metal sliding valve element and therefore, it is possible that certain complications could occasionally arise from this released piece of equipment. Further, the sleeve might hang up the bar or further slow its descent.
A combination bar actuated perforating gun and vent assembly is the subject of the present invention. The vent assembly is moved by the bar to the open position by the provision of a sliding valve assembly. The sliding valve assembly is moved and held firmly secured to the interior of the tubing at a predetermined location within the tubing string.